There's a common misunderstanding when talking about atheism that has reared its ugly head in the stoic philosophy forum I participate in. This post seeks to identify the problem and work toward greater understanding between those who believe and those who do not.
When an atheist describes his/her beliefs, he/she will typically use the phrase, "I do not believe in God."
When a theist argues with an atheist, he/she will typically use the phrase, "That atheist believes that there is no God."
The second statement is not an accurate translation of the first. Theism, in its many varieties, is a positive belief that there is a god or gods. Atheism is properly understood as a lack of this belief, not a positive belief in the non-existence of god.
As Dawkins notes in his book, The God Delusion, Christians are atheist with regard to Zeus, Vishnu, Tiamat, et al. The atheist just goes one god further.
The caricature of an atheist as someone who is sure that god does not exist makes it easy for the theist to beat up the straw man argument that both sides are equally unprovable. "I can't prove God exists, but you can't prove God doesn't exist." Then the theist will say that both positions are a matter of faith. The fallacy is that the theist is claiming that a deity exists without any evidence. A rational person suspends belief in the absence of evidence. I can't prove that leprechauns or the Flying Spaghetti Monster don't exist, but that doesn't make belief in them any less irrational.